The present invention relates generally to methods and apparatus for therapeutic treatment and/or monitoring of functions of the human or animal body, in part by means of the efficient transmission or delivery of electrical signals between a therapeutic or monitoring system, such as an automatic implantable cardioverter/defibrillator, and the tissue or blood of the body, such as that of the heart, via one or more implanted electrodes. More particularly, the invention is directed to improvements in the configuration, energy transfer efficiency, therapeutic and sensing effectiveness, and implantation techniques of such implantable electrodes, which in certain preferred applications are composed of carbon fibers or carbon-coated metallic fibers, for purposes of stimulating or sensing electrical reactions in body tissue and especially heart tissue.
As is observed in the '089 patent, the rapidly advancing state of the art of microelectronics has led to considerable progress in medical technology owing in great part to microminiaturization of electronic components and the consequent markedly reduced size of medical devices utilizing such components. This is particularly significant in the case of implantable medical devices of the types adapted to monitor and/or deliver electrical indicia and/or stimuli (broadly, electrical signals) within the body for the purpose of detecting or stimulating various selected biological processes. The devices of interest here are implantable and external defibrillators, which broadly include devices capable of monitoring and controlling cardiac activity as necessary to detect and alleviate arrhythmias or dysrhythmias such as fibrillation (both atrial defibrillation (AF) and ventricular fibrillation (VF)), pathologic tachycardia, and, in some of the multi-function devices, even bradycardia. In principle, these devices respond automatically to the sensing of the dysrhythmia of interest to restore normal, regular cardiac rhythm by delivering electrical stimuli in the form of pulses or shocks to the atrial or ventricular cardiac tissue, as appropriate.
Typically, the automatic defibrillator is implanted just beneath the flesh in the patient's abdomen or pectoral region, but the associated defibrillating or cardioverting electrodes have required implantation by a surgical procedure (thoracotomy) by which the chest cavity of the patient is opened to expose the heart and allow the electrode(s) to be secured directly to the epicardium or the pericardium. For example, one or more defibrillating patch electrodes of considerable surface area may be placed externally in stimulating relation to the myocardium so that an electric field may be established through the heart. Rather than applying patch electrodes in such a way that the electric field extends from one side of the heart to the other, usually through the ventricles, the epicardial patch or patches may be implanted to react with an endocardial counter-electrode implanted transvenously in the superior vena cava or further into the atrium or ventricle on the right side of the heart. The patch electrode(s) may be sutured directly to the epicardium after opening the pericardial sac. The implantation of electrodes external to the heart by the thoracotomy procedure is obviously highly traumatic to the patient, especially those with the very cardiac dysfunction it is intended to alleviate, and also necessitates considerable in-hospital recovery time for the patient.
Aside from risks and complications associated with the need for general anesthesia in such procedures, the electrode placement and accompanying thrombosis have tended to produce inefficient transfer of electrical energy between the body tissue and blood and the pulse generator of the defibrillator. Such inefficiency and the short to medium term increases in energy threshold commonly encountered with electrode implantation often combine to produce an inadequate result; that is, to retard the delivery of sufficient energy across the electron-ion interface, within the electrical energy capacity of the implanted and necessarily battery-powered device, to provide consistent and reliable defibrillation. As this is the primary function of the device, it becomes necessary to remove and replace the battery-depleted device (usually not exhausted but of inadequate stored energy), perhaps at altogether intolerably frequent intervals, with the attendant physical and psychological impairment of the patient.
Additionally, defibrillator implant patients have suffered inadequate healing, infection, pericardial hematomas and other trauma as a result of the surgical procedure, and some have experienced a further deterioration of the pumping function of the heart which is attributed to the sewn-on epicardial patch electrode. A survey of the various complications is treated in an article by D. Echt et al. titled "Clinical experience, complications and survival in 70 patients with the automatic implantable cardioverter/defibrillator", in Circulation, vol. 71, no. 2, 1985, pp. 289 to 296.
Prior art of interest is described in the following patents: in U.S. Pat. No. (USPN) 4,548,203, two pairs of spaced electrodes on the myocardium to control VF and tachyrhythmia at different times; in U.S. Pat. No. 4,481,953, an endocardial electrode with electrical connection between different low resistance conductors; in U.S. Pat. No. 4,355,646, an electrode arrangement for measuring impedance changes in the ventricle and for cardioversion using an endocardial electrode with two electrode points in each of the atrium and the ventricle connected by a low resistance triaxial lead to an implanted cardioverter or defibrillator; in U.S. Pat. No. 4,355,642, a disk electrode for sensing cardiac activity and for use in defibrillation; in U.S. Pat. No. 3,738,370, use of a bipolar coaxial catheter for defibrillation; in U.S. Pat. No. 4,708,145, an epicardial pad electrode and opposite endocardial electrode sequentially energized for defibrillation; in U.S. Pat. No. 4,787,389, endocardial and epicardial electrodes used for defibrillation and antitachycardia with means to prevent the antitachy pacemaker from damage by the defibrillating pulses; in U.S. Pat. No. 4,774,952, a multiple electrode structure concentrates current in selected areas of the heart during defibrillation. Other literature of interest includes Soviet author's certificate 1,263,260 with multi-electrode structure for even disposition about the heart to reduce current damage of myocardial cells; and British application 2,182,566, in which a disk electrode's elasticity permits it to conform to the heart's movements.
Further patents of interest include U.S. Pat. Nos. 4,270,549 and 4,291,707, in which a patch electrode is applied to the myocardium, a fine titanium wire structure is used as an electrode pole, and the mean current density is increased by using lateral insulators, and in which the patch electrode is applied without opening the chest cavity, by means of a spatula-like instrument introducible into the chest cavity through a cut, but of sizable dimensions--four to six centimeters (cm) wide and 1 to 3 cm thick--as to still require somewhat complex surgical procedures and use of general rather than local anesthesia.
Other patents representative of the state of the art in defibrillation and implantable defibrillating electrodes include U.S. Pat. Nos. 4,765,341 to M. Mower et al. and 4,512,351 to P. Pohndorf, and European application No. 0317490 of T. J. Fogarty published May 24, 1989 claiming U.S. priority date of Nov. 13, 1987.
Despite these proposals and some advances, problems have persisted in attempts to interface electrodes with body tissue to attain a combination of low resistance conductivity, large surface area, low polarization and low intrinsic stiffness, while providing long-term communication of electrical signals and relatively high current levels. Further, the implantation and manipulation of the prior art electrodes have remained complex procedures, and the methods and apparatus employed for defibrillation have not been entirely successful owing principally to inefficient energy transfer between the electrode and excitable tissue (i.e., tissue having cell membrane field strength which can be stimulated electrically to produce cell depolarization).
Energy transmission and transfer between pacemaker electrodes and the heart has received considerable attention. The reader's attention is invited, for example, to a survey article by Ripart and Muciga titled "Electrode heart interface: definition of the ideal electrode", in PACE, vol. 6, March 1983, pp. 410-421. Although there, some encouraging results were obtained using low polarization materials such as platinum, iridium and pyrolyzed carbon in the electrode tip having average surface area of 10 mm.sup.2 (square millimeters), for stimulation with pacing pulses ranging from 2.5 to 5.0 volts, and for sensing cardiac activity, cardiac pacing electrode requirements are manifestly different from defibrillation electrode requirements, the latter involving application of shocks with amplitudes of up to many hundreds of volts and over electrode surface areas of perhaps 10,000 mm.sup.2.
German Democratic Republic patent 26 32 39 of Oct. 30, 1987 discloses a pacing lead composed of a bundle of anisotropic carbon fibers. Such an electrode may be effective for pulse transmission along its longitudinal axis, but its relatively tiny point contact surface areas at the tissue interface and tendency of the electrode interface to erode over time would render it incapable of providing the large surface area required for defibrillation. The transfer of adequate energy from defibrillating pulses of from 500 to 2,000 volts typically requires patch electrode surface area of from 50 to 100 cm.sup.2 (square centimeters) to avoid possible local burning of the tissue, and transvenous electrode surface area of from 4 to 20 cm.sup.2 for uniform energy flow through the heart.
It is highly desirable that the defibrillating electrode have characteristics of low energy consumption, low polarization, flexibility to avoid mechanical restriction of movement of the heart during beating, and ease of implantation. It is further desirable that the defibrillating electrode be readily adapted for implantation without need for a thoracotomy and all its risks, patient recovery time, pericardial hemorrhaging, potential infection, and other complications such as those attributable to adhesions from prior surgery where the patient is undergoing electrode replacement. In addition to its advantages by way of reducing trauma and in-hospital recovery time, simplified implantation would reduce hospital costs otherwise associated the need for specialized open heart surgical facilities and attending personnel.
The invention disclosed in the '089 patent provides an improved lead assembly for biomedical use in stimulating and/or sensing electrical phenomena in relation to excitable tissue of the body. The electrode of the lead assembly is conductively connected to an electrical lead and is composed in principal part of flexible non-metallic electrically conductive fiber strands. The fibers possess substantially isotropic conductance characteristics to form an improved electrical interface with body fluid (e.g., blood) and/or excitable tissue when the lead assembly is implanted into the body. Specifically, the electrode characteristics improve the energy transfer and consequent electrical communication across the electron-ion interface between the electrode and the blood/tissue. This interface is formed along the entire length of exposed (uninsulated) surface of each fiber, rather than at restricted point contact areas. The isotropic property of the fiber material assures that it will conduct electrons substantially equally in all directions, thereby considerably expanding the electron-ion boundary which is otherwise substantially limited to such virtual point contact at the ends of the fibers in theretofore proposed carbon fiber electrodes. The electrode is intentionally constructed to provide a considerable length of uninsulated fiber surface to form the interface.
The '089 patent invention also provides a multiplicity of such fibers in the composition of the electrode of the lead assembly, which, together with the substantially isotropic conductance characteristic or property of each fiber and the exposure of a substantial length of the fibers in the electrode, presents an effective surface area which is much larger, indeed many times larger, than the apparent actual surface area of the electrode calculated from its linear dimensions. That invention also resides in a technique by which the fibers are held together to provide an electrode configuration having a size and shape adapted to form the desired interface according to the intended implant location of the electrode in the body. For example, the fibers may be woven in strands which themselves are interwoven to form two- or three-dimensional configurations especially suited as defibrillation electrodes. Further, the invention of the '089 patent provides methods of shaping and implanting such electrodes for various biomedical applications, including use with implantable automatic defibrillators.
The principal advantage of lead assemblies incorporating such electrodes over the prior art is the efficiency of energy transfer across the electrical interface at the electron-ion boundary between the flexible fiber electrode configuration and the body fluid and/or excitable tissue in contact with or in the immediate vicinity of the interface, both in terms of the proportion of electrical energy applied to the interface which is delivered across the interface, and the speed with which this delivery occurs. This is a function of the nature of the interface provided by the electrode in the lead assembly--low polarization, low capacitance, low resistance and low impedance--and the effective (in contrast to actual) surface area of the interface. Such advantages are particularly important in the case of implantable battery-powered medical devices, where wasted energy can cause rapid depletion of the batteries and consequent shorter intervals between surgical replacements. Worse, the battery depletion may render the device incapable of functioning properly in a life-threatening situation, such as in the case of a defibrillator implanted in a patient experiencing VF.
Tests have demonstrated that the energy yield (energy transfer) attained with an electrode of the type disclosed in the '089 patent, used for delivery of defibrillating shocks, is at least about 30% greater than that achieved with prior types of electrodes. Accordingly, successful defibrillation is much more likely at relatively lower levels of energy deliverable from the device, with the improved electrode. These tests have also shown that the integral of the pulse during even the first millisecond is about 50% greater (i.e., a much faster rise time for the delivered impulse of electrical energy) compared to lead assemblies with the best prior art electrodes, for the same absolute energy input. For example, a time interval of about 4-1/2 milliseconds was required to develop 14 joules of energy across the interface with a 30 joule input for conventional metal defibrillating electrodes, whereas electrodes fabricated according to the invention disclosed in the '089 patent yielded 22.6 joules for the same input and the same time interval.
The carbon fibers provide the electrode with a flexible configuration capable of conforming in shape and size to the region of tissue with which it is to interact, and allow the electrode to undulate with movements of the tissue such as during defibrillation. The fibers may be held together without bonding or other material which otherwise might to some degree adversely affect the size of the electrode's effective surface area relative to its apparent actual surface area for energy transfer.
The prior art relative to the invention disclosed in the '089 patent further includes:
U.S. Pat. No. 4,574,814 to Buffet, where an axially slidable coaxial probe provides contact in both atrium and ventricle in the form of carbon fiber feather dusters of variable separation for synchronous pacing in chambers of different size. The resiliently deformable fibers assume the feather duster shape when unrestrained, with the fiber ends providing an envelope of contact with myocardial tissue to increase the area of anchoring, and with reliance on end contact (point contact) of non-isotropic fibers with the tissue for excitation and anchoring.
An article by Starrenburg et al. titled "Carbon Fibers as an Electrode Material" in IEEE Transactions on Biomedical Engineering, vol. BME-29, no. 5 (May 1982), at pp. 352 et seq., which describes a flexible carbon fiber bundle electrode having a short segment of straight bare fibers between two insulated regions for muscle stimulation when implanted in the wall of the small intestine of a dog, but in which the carbon fiber electrode suffered breakdown during electrical pulse testing in a bath and as a result of mechanical stresses when implanted.
An article titled "New Plastics That Carry Electricity" in the Jun. 18, 1979 issue of Newsweek magazine, where polyacetylene plastic doped with chemicals to add free electrons are described for carrying electric current. The suggested uses include pacemaker lead wires, but no suggestion of forming the plastics into fibers or strands or of weaving fibers together or as fiber electrodes.
Great Britain patent No. 1 219 017 to Thomson Medical-Telco, in which an electrical conductor of braided carbonized fibers forms a lead for cardiac pacing, and is insulated along its entire length except at the very end to permit point contact excitation of tissue. The material is non-isotropic, of high resistivity for current flow in the direction of tissue or blood at the longitudinal surface of the fiber compared with the end point.
U.S. Pat. No. 4,506.673 to Bonnell describes electrical tissue growth stimulators using biodegradable electrically conductive carbon particle-impregnated cotton fibers in a non-isotropic mesh for cathodic and anodic stimulation.
U.S. Pat. No. 4,198,991 to Harris, in which a cardiac pacing lead uses carbon filaments covered with an insulating sheath except at the lead tip as a brush-like electrode structure for point contact of tissue.